They just don't care about another "HR" process.
But they do care about delivering results. So, focus your efforts there, with onboarding as a tool.
I learned the ABCs of managing behavior from Paul Brown a million years ago when I was at Procter & Gamble. His framework is that changing B-Behavior is best accomplished by changing A-Antecedents (what happens leading up to the behavior) and C-Consequences (what happens after the behavior).
Applying this framework, the argument is that you should invest in A) training managers on onboarding tools as a way to help them help their new employees deliver better results faster and C) provide consequences to managers related to their team's ability to deliver better results faster. If you do that, their B) onboarding behavior will change.
